The purpose of this study is to determine the safety and tolerability of PRTX-100 when various doses are given 5 times at weekly intervals to patients with active rheumatoid arthritis that are taking methotrexate or leflunomide. The drug is administered in a physician's office via an intravenous infusion. PRTX-100 may be effective in rheumatoid arthritis by suppressing the immune responses.